Thermionic tube and circuit



P 16, 1941- c. F. STROMEYEi? 2,256,177

THERMIONIC TUBE AND CIRCUIT Filed March 29, 1939 F1. 5' EH F i 5.31 1

l N\ E NTOR.

' ATTORNEYS (ha/[es Wane 1's Sim/weren- Pmma Sept. 16, 1941 THERMIONICTUBE AND CIRCUIT Charles Francis Stromeyer, East Orange. N. 3.,

assignor to Revelation: Patents Holding Company, New York, N. Y., acorporation of Dela- Application March 29, 1939, Serial No. 264,689

19 Claims.

This invention relates to thermionic tubes and especially to such tubesutilizing a space-charge electrode to provide a large virtual cathode.This invention also relates to the associated circuits for the purposeof amplifying, detecting, frequency converting signals, and to producelocal oscillatory energy with such tubes.

Since the amplifying capability of a thermionic tube is proportional toits plate transconductance, other factors being equal, there is aconstant quest to improve this control factor. To merely proportionatelyenlarge the tube elements is not the improvement intended here, for thatwould be like enlarging a given design of a motor which would result inincreasing its power but it does not necessarily follow that itsefficiency would be improved. This tube design problem usually resolvesto the question of obtaining greater transconductance per-mil platecurrent without increasing the cathode heating power and withoutappreciably increasing the structural size. I

A conventional method is to reduce the cathode to control grid clearanceand use a grid wire having the smallest diameter possible. It is obviousthat this method is limited by structural requirements such as neededrigidity and practicable clearance to avoid shorting of the tubeelements, by excessive variations in characteristics of tubes of a.given design, and by grid emission.

Another method is to provide a large so-called virtual cathode whicheffectively has much greater area than its associated real cathode. Thecontrol grid may be as close to the virtual cathode as desired with nodanger of shorting since the virtual cathode is an imaginary surfacesituated in space. Grid emission is minimized because the control gridis not close to the hot cathode. A virtual cathode can be readily formedby using a positive grid, known as a space-charge grid, preferablyadjacent to the cathode, that is between the cathode and control grid.The virtual cathode considered herein is an imaginary surface exhibitingcathode characteristics, located in the space between the space-chargegrid and the next succeeding positive grid, all points of which are atthe minimum space potential. By correctly positioning and proper designof the electrodes, a virtual cathode of large area may be had. Suchspace charge tubes can be made with very high ratio of transconductanceper-mil plate current. In spite of this, such tubes have not met withconspicuous commercial success. Probably the chief reason for this isbecause of the manufacturing difliculty of obtaining uniform tubecharacteristics among tubes of a given design. The percentage variationis much greater than with tubes of conventional construction not usingsuch virtual cathodes and its cause can only be partly attributed tomechanical displacement of the elements. My invention provides apractical solution to this basic diiiiculty.

Application of space-charge tubes to circuits is seriously limited dueto their relatively high inter-electrode capacities, particularly thecontrol electrode to anode. Another dimculty is the problem of obtaininga satisfactory sc-called variable-mu characteristic to provide a remoteplate current versus control grid voltage characteristic for minimizingcross modulation efiects, without seriously reducing thetransconductance. My invention also provides solutions to both of theseproblems. A tube to be later described in detail embodies theseteachings. This particular tube is intended for general radio frequencyamplification and detection applications. The effective grid-platecapacity of this tube is so low that conventional high Q input andoutput circuits may be used without inter-action between these circuits.A stage gain improvement of five times can be realized. This tube alsohas an overall remote plate current versus control grid voltagecharacteristic.

An object of this invention is to minimize variations in characteristicsof tubes utilizing a large virtual cathode by a special arrangement ofelectrode elements.

Another object of this invention is a method for coupling tube elementsso as to minimize inter-action between the associated input and outputcircuits.

Another object of this invention is a method for coupling tube elementsso as to minimize loading of the associated input circuit.

Still another object of this invention is a method for coupling tubeelements so as to provide an overall remote plate current versus controlgrid voltage characteristic without requiring a so-called variable-muaction with the tube elements that produce relatively hightransconductance.

In the drawings Fig. 1A is a schematic diagram showing an arrangement ofseveral electrodes according to the invention, and Fig. 1B is the sameschematic showing how it may be used in a simple amplifying circuit.

Fig. 2A and Fig. 3A are also schematic diagrid 4.

grams showing other arrangements of several electrodes according to theinvention, and Fig. 2B and Fig. 3B are the same schematics,respectively, showing how they may be used in a simple amplifyingcircuit.

Fig. 4 and Fig. 5 are also schematic diagrams showing two furtherarrangements of several electrodes according to the invention.

Fig. 6 is a schematic diagram showing a coupling arrangement of severalelectrodes and a simple amplifying circuit according to the invention.

Referring to Fig. 1A, an evacuated envelope 9 encompasses the tubesinter-electrodes. Cathode l, heated by heater 8, supplies a source ofemitted electrons. Grid I is a space charge grid and when properpositive potential is applied thereto, a virtual cathode is establishedsomewhere in the space between this grid and anode 6. Grid 2 is acontrol grid. A special grid 3 is connected to the control grid. Thisarrangement may be considered a triode type of a space charge tube withthe addition of the special grid, the action of which will be describedlater. Fig. 1B shows how this same tube may be used in a simpleamplifying circuit where the input signal is impressed across terminalsl2 and the output taken ofi terminals [3. Items III, II and 23 arepotential sources for operation of the device.

Reoccurring reference numerals in all the figo ures refer to likeelements and will not be repeatedly described. I

Fig. 2A and Fig. 2B are similar to Fig. 1A and Fig. 1B but with theaddition of screen grid 4. This arrangement may be considered a tetrode5 type of a space charge tube with the addition of the special grid 3.The virtual cathode in this tube is established som'ewhere inthe spacebetween the space charge grid I and the screen 40 Fig. 3A and Fig. 3Bare similar to Fig. 2A and Fig. 2B but with the addition of a suppressorgrid 5. This arrangement may be considered a pentode type of a spacecharge tube with the addition of the special grid 3.

Fig. 4 is similar to Fig. 3A but the special grid 3 is connected tocathode I instead of control grid 2. This modification may likewise beused with tubes arranged according to Figs. 1A and 2A. Fig. 5 is similarto Fig. 3A but the special grid 3 terminates independently of the othergrids instead of being connected to control grid 2, in order that it maybe connected at will to the control grid 2, cathode 'Lspace charge gridI, or to a low negative or positive potential with relation to thecathode. This modification may likewise be used with tubes arrangedaccording to Fig. 1A and Fig. 2A.

The word grid used herein is for illustration purpose and in no mannerlimits the invention to any particular physical structure of theelectrode. Any known arrangement of an electrode structure whichcontrols the number, velocity or direction of electrons travelingbetween cathode and anode may be applicable to the word grid as usedherein. v, I believe thetheory of how the special electrode justdescribed minimizes the variation in I transconductance of such devicesto be somewhat as follows: The position of the virtual cathode which isestablished somewhere in the space between the space-charge grid andanode or screen grid, may be considered at an imaginary surfacedescribed by al points that are at the minimum-space-potentia1 withinthe space.- In 75 virtual cathode.

the ideal case for maximum control, the control grid should be locatedjust beyond this imaginary surface, and it should provide a controllingfield which conforms to the configuration of this surface. In thepractical case, this imaginary surface is far removed from being asmooth plane and its configuration and position varies considerablyamong tubes of a given design. This results in variations oftransconductance-per-mil plate current. As the potential gradient on theanode or screen grid side of the control grid is lowered, the shape ofthe virtual cathode becomes more uniform and its position moreconsistent- In many instances low anode or screen voltage is notdesirable and often considerations of size limit the maximum permissibledistance between elements. The special grid located between the controlgrid and anode or screen grid can eflectively lower the potentialgradient immediately following the control grid without reducing thetransconductance-per-mil plate current. If the special grid is sodesigned with relation to the control grid that its efiect on the platetransconductance is much less than that provided by the control grid,and if it is connected to the control grid, the transconductanceper-milplate current may be higher than tubes of similar design but without thespecial grid. The special grid may be connected to the cathode or somelow positive or negative potential. However, when the special grid isconnected to the control grid an additional improvement in minimizingcharacteristic variation is realized.-

This can be explained by the fact that the depth of the controllingfield is increased, which makes the tube less critical to displacementand to irregularities in the shape of the virtual cathode. This methodof minimizing characteristic variations is indeed simple and highlyeffective, making it possible to manufacture very high transconductancetubes nearly commensurable with the percentage variation as in makingordinary triodes 0r pentcdes.

From the foregoing it is seen that a purpose of the special grid is toproduce a low potential gradient immediately following the virtualcathode. Another purpose is to increase the depth of signal controlregion directly following the It is evident that a compromised conditionis preferable because should the gradient beyond the virtual cathode bemade too flat, the control factor suffers. These statements may. seemvague but the art is' far too new to attempt mathematical expressionsfor optimum conditions in terms of parameters of tube elements andpotentials. It should be sufiicient to say that a purpose of the specialgrid is to produce a low potential gradient for a substantial distanceimmediately following the virtual cathode. In the case of a medium sizetube, this distance might be .025" and the average slope may be only afraction of the voltage on the succeeding positive electrode. It alsoshould be sumcient to say that a purpose of the special grid is toincrease the depth of the signal control region immediately followingthe virtual cat ode. In the case of a medium size tube, this depth mightbe .025".

In order to avoid unnecessary experimentation by those skilled in theart to determine how my invention may be applied in a practicalapplication, I-will now set forth several specific details of onedesign. The tube in question is arranged like Fig. 3A and measured in acircuit like Fig. 33. An indirectly heated electron emitting cathode.030" in diameter is surrounded by concentric grids of the helical type,each supported at their major diameter by two lateral posts. Certaindetails of these grids follow:

A circular plate of .910" diameter surrounds the grids. These tubeelements are supported in such manner as to be insulated from each otherexcept the number of terminating leads are lessened by internallyconnecting grid #3 to grid #2, and grid #5 to cathode. The effectivelength of these elements is .800" and the cathode coating band length is.590". It is preferable to shield the plate in such manner as tominimize electronic bombardment of the envelope. In order to readilyevaluate the improvement in plate transconductance compared with one ofthe most popular standard contemporary pentodes made in this country,known as type #SK'IG, the following figures are given:

The figures for the type GK'ZG are taken from its published rating andfigures for the special tube are approximate average values taken from aquantity of tubes made according to this design. The improvement intransconductance is approximately eight times. The improvement intransconductance per mil-plate current is approximately fourteen times.The improvement in tranconductance per-mil total cathode current isapproximately eight times. The actual space between the cathode sleeveand #1 grid wire is .058" for the new tube; and for the 6K'IG, whosesleeve is usually .045 diameter and minorjtl grid diameter .080", isactually .0145". This is a four times improvement in cathode to #1 gridclearance.

Tubes of this type, however, have disturbingly high effectiveinter-electrode capacities. The limitation of amplifying tubes,particularly with relation to grid-plate capacity, is well recognized.

Since this capacity appears to the associated circuits effectively as avalue equal to the gridplate capacity times the gain of the tube, it isapparent that the general usefulness of high transconductance tubes islimited unless this capacity is made extremely small or nullified. Alsothe high inter-electrode capacities and transconductance cause the inputadmittance to be high, resulting in a further limitation due to loadingof the input circuit. These difliculties may readily be overcome byemploying a special coupling arrangementof tube elements asshowninFlg.6.

Referring to Fig. 6, evacuated envelope 9 encompasses two sets ofelectrodes, comprising a driver set having a cathode l4, signal controlgrid l5, and anode II, and an output set having the same electrodearrangement as described in Fig. 3A. Cathode i4 is connected internallyto grids 2 and 3, anode I6 to grid 4, and grid 5 to cathode I. Theseinternal connections lessen the number of base terminals required andminimize stray lead capacity and inductance. Cathode Il returns to anegative pole of the potential source II by the series impedance l'l.Element I1 is represented as an impedance for 11- lustrative simplicityand, of course, it may be any type of a known series load, tuned oruntuned. Cathode l returns to a negative pole of the potential source IIby the series impedance l8, andthis element is by-passed by condenserl9. Bias voltage for the grids l5 and 2 are provided by the voltagedrops across the elements 11 and 18. This coupling of an input set ofelements to an output set, and bias arrangements, are similar to myUnited States Patent No. 2,154,783. Any known input device may beconnected across terminals 12 to impress the input signal between gridl5 and a negative end of the potential source ll. Any known outputdevice may be connected between terminals 13 to provide a work loadbetween anode 6 and a positive end of the potential source It. Ratherthan show'the potential feeds for grid. l and grid 4 as merely taps fromthe potential source ll, one practical method of supplying the gridsfrom one point of potential is shown. Grid returns to the potentialsource via resistors 2| and 22. Grid 4 returns to the potential sourcevia resistor. 22 which is by-passed to cathode 'l by condenser 20 inorder to make the potential on grid 4 substantially constant. If thetube is correctly designed no loss is suffered due to the resistor 2|being un-by-passed. In fact, with some designs, an increase in gain willresult if resistor 21 is left un-by-passed. Heater 8 renders cathodes Iand I4 emissive.

The advantages'of this coupling arrangement are many. Due to thedegenerative repeating action of the driver, resulting in a gain lessthan unity, its effective input admittance as it appears to the inputdevice is very low. Since the input of the output section is shunted bya relatively low impedance element I'I, its high input admittance is nolonger troublesome. The result is that the driver section effectivelyisolates the impressed input circuit from the output load circuit so asto reduce inter-action between these circuits to a' minimum. Also theloading of the impressed input circuit is very small. When it isdesirable to have a remote overall plate current versus control gridvoltage characteristic to minimize cross modulation, it is not necessaryto use a so-called variable-mu action in the output section for thedesired characteristic may be obtained by proportioning the platecurrent cut-off rate of the individual sections and by proper selectionof elements H and I8. This flexibility permits the use of a sharp platecurrent cut-off output section and a variable-mu type driver. Theadvantage of this arrangement is that a much higher overalltr/ansconductance-per-mil plate current can be obtained and -with betterconsistency of characteristics.

It is obvious from the foregoing that it is not essential to have thetwo sections of tube elements in one envelope. Separate tubes mayreadily be used. It is also obvious that this coupling arrangement isnot limited to repeating signals of a particular band of frequencies, orit may be used to repeat impulses of audio or radio frequency. Thiscoupling arrangement is not limited solely to amplifying signals for itmay also be used for detector and frequency converter signals and toproduce local oscillatory energy.

What I claim is:

1. A thermionic tube having a cathode capable of emitting electrons andan anode, a spacecharge grid capable of producing a virtual cathode inthe space between the space-charge grid and anode when a positivepotential is applied to the space-charge grid, at least onesignalcontrol grid between the space-charge grid and anode, an auxiliarygrid between the signal-control grid and anode, a direct connectionbetween the signal-control grid and the said auxiliary grid.

2. A thermionic tube having a cathode capable of emitting electrons andan anode, a spacecharge' grid capable of producing a virtual cathode inthe space between the space-charge grid and anode when a positivepotential is applied to the space-charge grid, at least onesignalcontrol grid between the space-charge grid and anode, an auxiliarygrid directly between the signal-control grid and anode, a directconnection between the'auxiliary grid and cathode.

3. A thermionic tube having a cathode capable of emitting electrons, ascreen grid, and an anode, a space-charge grid capable of producing avirtual cathode in the space between the spacecharge grid and screengrid when a positive potential is applied to the space-charge grid, atleast one signal-control grid between the spacecharge grid and screengrid, an auxiliary grid between the signal-control grid and thescreengrid, a direct connection between the signalcontrol grid and thesaid auxiliary grid.

4. A thermionic tube according to claim 3 with the addition of 'asuppressor grid located between. the screen grid and anode.

5. A thermionic tube having a cathode capable of emitting electrons, ascreen grid, and an anode, a space-charge grid capable of producing avirtual cathode in the space between the spacecharge grid and screengrid when a positive potential is applied to the space-charge grid, atleast one signal-control grid between the spacecharge grid and screengrid, an auxiliary grid be ween the signal-control grid and the screengr (1, a direct connection between the auxiliary grid and cathode.

6. A thermionic tube according to claim 5 with the addition of asuppressor grid located between the screen grid and anode.

'7. A thermionic tube having a cathode capable of emitting electrons, ascreen grid, and an anode, a space-charge grid capable of producing avirtual cathode in the space between the spacecharge grid and screengrid when a positive potential is applied to the space-charge grid, at

least one signal-control grid between the space-.

charge grid and screen grid, an auxiliary grid between thesignal-control grid and anode to produce a lower potential gradient inthe space immediately following the signal-control grid than there wouldbe if the said auxiliary grid were not present.

8. A thermionic tube according to claim 7 with the addition of asuppressor grid located between the screen grid and anode.

9. A thermionic tube having a cathode capable of emitting electrons andan anode, a screen electrode capable of accelerating the electronspass.- ing through or around the screen electrode when a positivepotential is applied thereto, a spacecharge electrode capable ofestablishing a virtual cathode between this electrode and the screenelectrode when a positive potential is applied to the space-chargeelectrode, a control electrode located in the space between thespace-charge electrode and the screen electrode capable of controllingthe number of electrons arriving at the anode when signal impulses areapplied to the signal-control grid, at least one additional electrode inthe space between the space-charge grid and screen electrode to producea. lower potential gradient in the space immediately following thesignal-control electrode than there would be if the said additionalelectrode were not present. I

10. A thermionic tube having a cathode capable of emitting electrons andan anode, a screen electrode capable of accelerating the electronspassing through or around the screen electrode when a positive potentialis applied thereto, a space-charge electrode capable of establishing avirtual cathode between this electrode and the screen electrode when apositive potential is applied to the space-charge electrode, a controlelectrode located in the space between the space-charge electrode andthe screen electrode capable of controlling the number of electronsarriving at the anode when signal impulses are applied to thesignal-control grid, at least one additional electrode in the spacebetween the space-charge grid and screen electrode tofurther control thenumber of electrons arriving at the anode when signal impulses areapplied to the said additional electrode and to produce a lowerpotential gradient in the space immediately following the signal-controlelectrode than there would be if the said additional electrode were notpresent.

11. A thermionic tube comprising a driver-set of electrodes and anoutput set of electrodes, the driver set including at least an electronemitting cathode, a signal-control electrode and an anode; theoutput'set including an electron emitting cathode and an anode, aspace-charge electrode capable of establishing a virtual cathode betweenthis space-charge electrode and the second mentioned anode, a controlelectrode located between the said space-charge electrode and the secondmentioned anode, at least one additional electrode located in'the spacebetween the said spacecharge electrode and the anode to produce a lowerpotential gradient in the space immediately following the said controlelectrode than there would be if the said additional electrode were notpresent; a direct electrical connection between the first mentionedcathode and the said control electrode.

12. A thermionic tube according to claim 11 in which the driver set ofelectrodes are of the variable mu type providing a remote anode currentversus signal-control electrode characteristic.

13. A thermionic tube having a driver set of electrodes and an outputset of electrodes, the driver set including at least an electronemitting cathode, a signal-control electrode and an anode; the outputset including an electron emitting cathode, a screen electrode, ananode, a space-charge electrode capable of establishing a virtualcathode between this space-charge electrode and the screen electrodewhen a positive potential is applied to this space-charge electrode, acontrol electrode located between the space-charge electrode and thescreen electrode, and at least one additional electrode adjacent to thesaid control electrode, a direct electrical connection joining the firstmentioned cathode to the said control electrode and to the saidadditional electrode, and another direct electrical connection joiningthe first mentioned anode to the said screen grid.

14. A thermionic tube according to claim 13 connected in a waverepeating circuit comprising a signal input device connected between thesignal-control electrode and ground, an output device connected betweenthe anode of the output sectionand a positive pole of a potentialsource, a load element connected between the cathode of the driver andground, a connection between the cathode of the output section andground, positive potentials applied to both the 'space-chargeelectrodeand the screen electrode.

15. A thermionic tube according to claim 13 connected in a waverepeating circuit comprising a signal input device connected between thesignal-control electrode and a negative pole of a potential source, anoutput device connected between the anode oi the output section and apositive pole of the said potential source, a load element connectingthe cathode of the driver to a negative pole of the said potentialsource, a connection between the cathode of the output section and anegative pole of the said potential source, a resistor connected betweenthe spacecharge electrode and a positive pole of the said potentialsource, a connection between the screen electrode and an intermediatetap on the said resistor, and a condenser shunting the said tap toground or to a point at the minimum signal source, and positivepotentials applied to both the space-charge electrode and the screenelectrode.

17. A thermionic tube according to claim 13 connected in a waverepeating circuit comprising a signal input device connected between thesi nal-control electrode and a negative pole of a potential source, anoutput device connected between the anode of the output section and apositive pole '01 said potential source, a load element connecting'thecathode of the driver to a negative pole of said potential source, aconnection charge electrode and the screen electrode.

18. In an electrical system, a first thermionic tube, a secondthermionic tube; said. first tube having an electron emitting cathode,at least a signal-control electrode and an anode; said second tubehaving a space charge electrode, a control electrode, an anode, and atleast one additional electrode positioned to make the potential fieldimmediately following the said control electrode less than it the saidadditional electrode were not present; a short circuit connectionbetween the cathode of the first tube and the said control electrode; asource of anode potential supply; means for connecting the anodes ofeach ofv said valves to a positive terminal of said source; a circuitconnecting the cathode of the first valve to a negative terminal of saidsource and including a load element in series; a circuit connecting thecathode of the second tube with a negative terminal of said source,whereby potential tor biasing the control electrodes-is obtained byutilizing volta e existing between the cathode of each valve and anegative end 0! said source; a circuit connecting the saidsignal-control electrode to a negative terminal of said source andincluding in series circuit elements ior introducing an input signal;and means for connecting the said space-charge electrode to a positiveterminal of said source.

19. In an electrical system according to claim 18 in which the elementsof the first thermionic tube are arranged to provide a variable mucharacteristic so that the trans-conductance of the second tube can begradually controlled by vary-- ing a negative potential in series withthe circuit connecting the signal-control electrode or the first tube toa negative terminal or the source of anode potential supply.

CHARLES FRANCIS S'I'ROMEYER.

